The present invention relates to apparatus for handling sheets of print receiving material during a print operation and, more particularly, to a paper handling system for an ink jet printer in which sheets of print receiving medium are successively loaded onto a sheet supporting drum which is rotated adjacent a print head. The sheets are held on the drum by vacuum applied to an interior vacuum cavity in the drum which communicates with vacuum openings in the drum surface.
Such a printer is disclosed in U.S. Pat. No. 4,106,061, issued Aug. 8, 1978, to Burnett. The Burnett patent discloses a paper supporting drum upon which is mounted a sheet of copy paper. The drum has associated therewith an ink jet printer which is moved parallel to the axis of rotation of the drum during a plurality of drum rotations as scanning is performed in a corresponding manner across an original document. A plurality of vacuum belts cooperate with evacuated chambers beneath the belts to transfer copy paper from a supply tray to the drum for printing and, after the printing operation is completed, to transfer the printed copies to a storage tray. Printing is accomplished during a plurality of rotations of the paper supporting drum during which time the ink jet printer prints in a helically interlaced pattern on the copy paper.
In a number of prior art printers or copiers, copy paper is mounted on a rotating drum during printing by means of a mechanical sheet engaging mechanism which grips one or more of the edges of the sheet of copy paper. Such an arrangement is shown in U.S. Pat. No. 4,033,575, issued July 5, 1977, to Fujimoto. Although the mechanical sheet grippers provide positive location of the sheet of copy paper on the drum, such mechanical sheet engaging arrangements are necessarily limited in operating speed and may also be relatively complicated and subject to wear and malfunction. Additionally, such arrangements are susceptible of improper operation with the result that one or more sheets of copy paper may become jammed in the mechanism, requiring operator intervention for removal of the sheets.
The use of drums defining vacuum openings in the periphery thereof for engaging a sheet of copy paper by means of a sheet engaging suction is shown in a number of prior art references including U.S. Pat. No. 4,101,018, issued July 18, 1978, to Sokolowski. In the Sokolowski patent, a paper engaging drum defines a plurality of suction openings which are spaced uniformly about the periphery of the drum. The drum is sized such that the trailing edge of a sheet of paper loaded thereon overlaps the leading edge slightly. A photo-optical sensor arrangement senses this overlap in order to provide control signals to the printer such that the image is printed on the sheet of paper in proper registration. The sensor arrangement is required in the Sokolowski device since positioning of a sheet of paper at a precisely defined position on the vacuum drum is not possible.
U.S. Pat. No. 3,845,951, issued Nov. 5, 1974, to Hamaker, discloses a vacuum roller having a coating of compressible porous material which defines a registration ridge extending axially along the roller surface. A sheet of paper is transferred into contact with the roller such that the leading edge abuts this ridge. This arrangement provides for positive registration of the sheet on the roller. Such a roller is not acceptable as a support for a sheet of copy paper during printing with an ink jet printer, however, since the locating ridge would contact the printer as it is rotated therepast.
Accordingly, it is seen that there is a need for a vacuum drum mounting arrangement for supporting a sheet of copy paper as an image is printed on the sheet with an ink jet printer in which registration of the sheet of paper on the drum is assured and in which the sheet of paper is removed from the drum when improperly positioned on the drum or when the drum vacuum is lost.